LU-7435 test: managing OST pools properly

LU-7435 test: managing OST pools properly

ost-pools tests fail on configuration MGS/MDS not combined,
different nodes, configured as failover pair.

Patch fixes ost-pools to execute commands on mgs instead of
$SINGLEMDS.

Seagate-bug-id: MRP-2733
Signed-off-by: Elena Gryaznova <elena.gryaznova@seagate.com>
Change-Id: I8dfe4cc38a30af59f6cf5f4dcd144588b499c3c3
Reviewed-on: http://review.whamcloud.com/17326
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Nunez <james.a.nunez@intel.com>
Reviewed-by: Wei Liu <wei3.liu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>